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arXiv:astro-ph/0208556v3 23 Sep 2002 Galactic Environment of the Sun and Stars: Interstellar and Interplanetary Material By Priscilla C. Frisch and Hans R. M¨ uller AND Gary P. Zank and C. Lopate 1 University of Chicago, Chicago, IL 2 Bartol Research Institute, University of Delaware, Newark, DE 3 IGPP, University of California, Riverside, CA 4 University of Chicago, Chicago, IL Interstellar material surrounding an extrasolar planetary system interacts with the stellar wind to form the stellar astrosphere, and regulates the properties of the interplanetary medium and cosmic ray fluxes throughout the system. Advanced life and civilization developed on Earth during the time interval when the Sun was immersed in the vacuum of the Local Bubble and the heliosphere was large, and probably devoid of most anomalous and galactic cosmic rays. The Sun entered an outflow of diffuse cloud material from the Sco-Cen Association within the past several thousand years. By analogy with the Sun and solar system, the Galactic environment of an extrasolar planetary system must be a key component in understanding the distribution of systems with stable interplanetary environments, and inner planets which are shielded by stellar winds from interstellar matter (ISM), such as might be expected for stable planetary climates. 1. Introduction Our solar system is the best template for understanding the properties of extrasolar planetary systems. The interaction between the Sun and the constituents of its galactic environment regulates the properties of the interplanetary medium, including the influx of interstellar matter (ISM) and galactic cosmic rays (GCR) onto planetary atmospheres. In the case of the Earth, the evolution of advanced life occurred during the several million year time period when the Sun was immersed in the vacuum of the Local Bubble (Frisch and York 1986, Frisch 1993). Here we use our understanding of our heliosphere to investigate the astrospheres around extrasolar planetary systems.The heliosphere, or solar wind bubble, is dominated by interstellar matter (a visual- ization of the heliosphere is shown in Fig. 1). Interstellar gas constitutes 98% of the diffuse material in the heliosphere, and the solar wind and interstellar gas densities are equal near the orbit of Jupiter, beyond which the ISM density dominates. The solar wind and photoionization prevents nearly all ISM from reaching the Earth. Interstellar ions and the smallest interstellar dust grains(<0.1 µm) are deflected around the heliosphere. Neutral ISM, however, enters the heliosphere where it dominates the interplanetary envi- ronment throughout most of the heliosphere, except for the innermost regions where the solar wind dominates. Inner and outer planets experience radically different exposures to raw ISM over the lifetime of a planetary system. The exposure levels of the Earth to galactic cosmic rays and raw and processed ISM depends sensitively on heliospheric properties. Longstanding theories suggest that interstellar material has the potential to modify the terrestrial climate. These theories have recently become less speculative because of the improved understanding of cosmic ray modulation in a time-varying heliosphere and This paper is based on the talk presented at the Space Telescope Science Institute May, 2002 Symposium on the “Astrophysics of Life”. See “Interstellar and Interplanetary Material”, linked to http://ntweb.stsci.edu/sd/astrophysicsoflife/index.html. 1

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Page 1: Galactic Environment of the Sun and Stars: Interstellar ... · Galactic Environment of the Sun and Stars: Interstellar and Interplanetary Material By Priscilla C. Frisch and Hans

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Galactic Environment of the Sun and Stars:Interstellar and Interplanetary Material

By Priscilla C. Frisch and Hans R. Muller AND Gary P. Zank and C.Lopate

1University of Chicago, Chicago, IL2Bartol Research Institute, University of Delaware, Newark, DE

3IGPP, University of California, Riverside, CA4University of Chicago, Chicago, IL

Interstellar material surrounding an extrasolar planetary system interacts with the stellar windto form the stellar astrosphere, and regulates the properties of the interplanetary medium andcosmic ray fluxes throughout the system. Advanced life and civilization developed on Earthduring the time interval when the Sun was immersed in the vacuum of the Local Bubble andthe heliosphere was large, and probably devoid of most anomalous and galactic cosmic rays. TheSun entered an outflow of diffuse cloud material from the Sco-Cen Association within the pastseveral thousand years. By analogy with the Sun and solar system, the Galactic environment ofan extrasolar planetary system must be a key component in understanding the distribution ofsystems with stable interplanetary environments, and inner planets which are shielded by stellarwinds from interstellar matter (ISM), such as might be expected for stable planetary climates.

1. Introduction

Our solar system is the best template for understanding the properties of extrasolarplanetary systems. The interaction between the Sun and the constituents of its galacticenvironment regulates the properties of the interplanetary medium, including the influx ofinterstellar matter (ISM) and galactic cosmic rays (GCR) onto planetary atmospheres.In the case of the Earth, the evolution of advanced life occurred during the severalmillion year time period when the Sun was immersed in the vacuum of the Local Bubble(Frisch and York 1986, Frisch 1993). Here we use our understanding of our heliosphereto investigate the astrospheres around extrasolar planetary systems.†The heliosphere, or solar wind bubble, is dominated by interstellar matter (a visual-

ization of the heliosphere is shown in Fig. 1). Interstellar gas constitutes ∼98% of thediffuse material in the heliosphere, and the solar wind and interstellar gas densities areequal near the orbit of Jupiter, beyond which the ISM density dominates. The solar windand photoionization prevents nearly all ISM from reaching the Earth. Interstellar ionsand the smallest interstellar dust grains(<0.1 µm) are deflected around the heliosphere.Neutral ISM, however, enters the heliosphere where it dominates the interplanetary envi-ronment throughout most of the heliosphere, except for the innermost regions where thesolar wind dominates. Inner and outer planets experience radically different exposuresto raw ISM over the lifetime of a planetary system. The exposure levels of the Earthto galactic cosmic rays and raw and processed ISM depends sensitively on heliosphericproperties.Longstanding theories suggest that interstellar material has the potential to modify

the terrestrial climate. These theories have recently become less speculative because ofthe improved understanding of cosmic ray modulation in a time-varying heliosphere and

† This paper is based on the talk presented at the Space Telescope Science Institute May,2002 Symposium on the “Astrophysics of Life”. See “Interstellar and Interplanetary Material”,linked to http://ntweb.stsci.edu/sd/astrophysicsoflife/index.html.

1

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2 Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material

Figure 1. The Galactic Environment of the Sun – Upstream viewpoint: Visualization of helio-sphere moving through our galactic neighborhood, based on an MHD simulation of the helio-sphere morphology which includes the relative orientation and ram pressures of the interstellarand solar wind magnetic fields due to the ecliptic tilt with respect to the galactic plane (Lindeet al. 1998). There is a north-south asymmetry in the heliosphere from the ecliptic tilt withrespect to the interstellar magnetic field. The Mach ∼1 bow shock around the heliosphere isapparent, as is the termination shock of the solar wind (the smaller rounded surface inside ofthe heliopause where the solar wind transitions to subsonic). This figure is excerpted from amovie showing a 3D visualization of the heliosphere and the Milky Way Galaxy, which can beviewed at http://cs.indiana.edu/∼soljourn.

the relation between cosmic rays fluxes and atmospheric electricity and troposphericcloud cover (Section 6). Extrasolar planetary systems are surrounded by astrospheresformed by the interaction between stellar winds and interstellar material. In turn, theseastrospheres modulate the entrance and transport of galactic cosmic rays, anomalouscosmic rays, neutral interstellar (IS) atoms, and IS dust into and within the planetarysystem. Planet habitability has been evaluated in terms of atmospheric chemistry andenergy budget (see other papers in this volume). However by analogy with the solarsystem, an historically stable astrosphere may also be a predictor for stable planetaryclimates and thus the conditions which promote the development of advanced life. It isthis relation between the galactic environment of a star, the stellar astrosphere, and theproperties and prehistory of the interplanetary medium of planetary systems that are ofthe greatest interest.

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Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material 3

2. Heliosphere and Interstellar Matter

The heliosphere is the region of space filled by the solar wind, which is the expandingsolar corona. The solar wind corresponds to a solar mass loss rate of ∼10−14 MSun year−1.The solar wind density decreases with R−2 as the solar wind expands, and the solar windand interstellar medium pressures are equal at a plasma contact discontinuity known asthe “heliopause” (e.g. Axford 1972, Holzer 1989). The basic properties of the heliosphereare shown in Fig. 2 (Zank et al. 1996). At the solar wind termination shock the solarwind becomes subsonic and the cool supersonic solar wind plasma is shock-heated to ahot (T∼ 2× 106 K) subsonic plasma. Interstellar neutrals cross the plasma regions withinteraction mean free paths ∼100 AU. If the relative Sun-cloud velocity (26 km s−1)exceeds the fast magnetosonic speed of the surrounding interstellar cloud, a bow shockwill form around the heliosphere.

Solar wind properties vary with the 22-year magnetic activity cycle of the Sun, withthe solar magnetic polarity changing every 11 years during the period of the maximum insolar activity. During solar minimum, high speed low density solar wind forms in coronalholes at the solar poles (n(p+) ∼ 2.5 cm−3, velocity V ∼ 770 km s−1, McComas et al.2001). During solar maximum conditions, high speed stream material expands to theequatorial regions and the 1 AU ecliptic solar wind properties are: density n(p+) ∼4–8 cm−3, velocity V ∼350–750 km s−1, and magnetic field B ∼2 nT (or 20 µG). Theactivity cycle of the Sun is known to produce small modifications in the heliosphere overthe 11-year solar cycle, with the termination shock moving outwards ∼10 AU in theupwind direction, and outwards by ∼40 – 50 AU in the downstream direction duringsolar minimum.

The Sun is presently in a low density, warm, partially ionized interstellar cloud withnH∼0.24 cm−3, n(e−)∼0.1 cm−3, and T∼6,500 K (Slavin and Frisch 2002). The up-stream direction of the surrounding cloud, known as the Local Interstellar Cloud (LIC),is towards lII=3.3o, bII=+15.9o (in the rest frame of the Sun) and the relative Sun-LICvelocity is 26.4±0.5 km s−1 (Witte, private communication). The LIC upstream direc-tion in the local standard of rest (LSR, after removing the solar apex motion) is l=346o,b=–1o with a LIC velocity through the LSR of –15 km s−1. The LIC is a member of acluster of cloudlets flowing at –17±5 km s−1 from the LSR upstream direction of lII=2o,bII=–5o (Frisch et al. 2002). The LSR upstream direction is sensitive to the assumedsolar apex motion.†

The present-day Galactic environment of the Sun yields a highly asymmetrical helio-sphere that is much larger than the planetary system. A range of multifluid, Boltzmann-kinetic, and MHD models of the heliosphere has been developed (see Zank, 1999, for areview). In the upstream direction, the solar wind termination shock (where the solarwind becomes subsonic) is at about 75–90 AU. The heliopause is located near 140 AUand represents the contact discontinuity between the solar wind and interstellar plasmacomponent. The Sun is moving supersonically with respect to the LIC (sound speed is∼10 km s−1), however a weak interstellar magnetic field (∼3 µG, fast mode velocity ∼23km s−1) may yield a barely supersonic heliosphere (M∼1) with a bow shock. Severalheliosphere models place a weak bow shock at ∼250 AU in the upstream direction (seeZank 1999). For comparison, the planet Pluto is at 39 AU, and the Voyager 1 and Voy-ager 2 spacecraft are at 84 AU and 65 AU, respecively. In the downstream direction, thetermination shock is elongated by a factor of ∼2 compared to the upstream direction.

† These quoted values use a solar apex motion derived from Hipparcos data (Dehnen Binney1998). The basic solar apex motion yields the LIC LSR upstream direction lII∼326o, bII∼+4o

(Frisch 1995).

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4 Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material

Figure 2. This figure displays the neutral hydrogen density (bottom panel) and plasma tem-perature (top panel) of the heliosphere immersed in the LIC, which has properties T∼6,500 K,n(Ho)∼0.24 cm−3, n(H+)∼0.1 cm−3, and an unknown but probably weak magnetic field. Thehydrogen wall is formed by charge exchange coupling between weakly decelerated and deflectedinterstellar protons, and interstellar Ho.

The north ecliptic pole points towards the galactic coordinates l=96o, b=+30o, so theecliptic plane is inclined by ∼60o with respect to the plane of the galaxy. A pronouncedasymmetry between the northern and southern ecliptic is predicted for the heliospherebecause of this tilt and the LIC upstream direction (e.g. Linde et al. 1998), combinedwith the likelihood that the localized interstellar magnetic field is in the galactic plane(Frisch 1990).

Interstellar plasma piles up against the compressed solar wind in the outer heliosphere,and charge-coupling between interstellar Ho and interstellar H+ produces a low columndensity (N(Ho)∼ 3 x 1014 cm−2), decelerated (δV∼8 km s−1), heated (∼29,000 K) Ho

component that is visible as a redshifted shoulder in the Lyα absorption profile towardsα Cen (Linsky Wood 1996, Gayley et al. 1997, the “hydrogen wall”). Similar pileups ofinterstellar Ho have been detected against the astrospheres around several nearby coolstars (Section 5).

The charged component of the ISM is deflected by the tightly wound solar wind mag-netic field in the heliosheath region. The smallest interstellar dust grains(<0.1 µm) arealso deflected around the heliopause (Frisch et al. 1999). Neutral ISM, however, enters the

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Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material 5

heliosphere where it dominates the interplanetary environment throughout most of theheliosphere, with the exception of the innermost regions where the solar wind dominates.The Voyager 1 and Voyager 2 spacecraft are sending back data from the frontiers

of the outer heliosphere, and future spacecraft may penetrate interstellar space (e.g.,the Interstellar Probe mission, Liewer and Mewaldt 2000) and provide the first in situ

measurements of the galactic environment of the Sun. These spacecraft, and others (e.g.Ulysses, Galileo, Cassini) have provided a wealth of data which clearly demonstrate thatthe ISM dominates the interplanetary environment throughout most of the solar systemand heliosphere.

3. Historical Variations of the Heliosphere

The Galactic environment of the Sun and stars vary with the motions of the stars andinterstellar clouds through space. The Sun itself has been immersed in the vacuum ofthe Local Bubble (n(Ho)<0.0005 cm−3, n(H+)∼0.005 cm−3, and T∼106 K) during themillions of years over which homo sapiens developed and civilization emerged (Frisch andYork 1986, Frisch 1993). The Sun has recently (2,000 – 105 years ago) entered an outflowof diffuse ISM from the Sco-Cen Association (Frisch 1994, Frisch et al. 2002), and is nowsurrounded by a warm low density partially ionized cloud. The Sun may encounter otherpossibly denser cloudlets in the flow, with one possibility being the “Aql-Oph” cloudletthat is within 5 pc of the Sun near the solar apex direction. A study of nearby ISM shows96 interstellar absorption components are seen towards 60 nearby stars sampling ISMwithin 30 pc (Frisch et al. 2002). Since the nearest stars show ∼1 interstellar absorptioncomponent per 1.4-1.6 pc, relative Sun-cloud velocities of 0-32 km s−1 suggest variationsin the galactic environment of the Sun on timescales <50,000 years.The galactic environment of an astrosphere has a striking effect on the resulting as-

trosphere. This is illustrated in Fig. 3 for the heliosphere, which shows the heliosphereproperties several million years ago when the heliosphere was embedded in the LocalBubble (left), and at some time in the future when it might be embedded in a cloudwith density n(Ho)=15 cm−3 (but otherwise like the LIC). During the time the Sun wasembedded in the fully ionized Local Bubble Plasma, described by T=106 K, n(p+)=0.005cm−3, there were no interstellar neutrals in the heliosphere, and hence very few pickupions or anomalous cosmic rays (very small quantities of each may have been presentfrom a poorly understood inner source that may be related to either interplanetary dustor outgassing from planetary atmospheres). An increase to n=10 cm−3 for the cloudaround the Sun would contract the heliopause to radius of ∼14 AU, increase the den-sity of neutrals at 1 AU to 2 cm−3, and create a Rayleigh-Taylor unstable heliopausefrom variable mass loading of solar wind by pickup ions (Zank & Frisch 1999). Modelswith higher densities (e.g. n=15 cm−3, T=3,000 K) show that planets beyond ∼15 AU(Uranus, Neptune, Pluto) will be outside of the heliosphere for moderate density diffuseclouds, and thus exposed to raw ISM. The Sun is predicted to encounter about a dozengiant molecular clouds, with much higher densities (>103 cm−3) over its lifetime (Talbot& Newman 1977), but encounters with diffuse clouds (n ∼10 cm−3) will occur morefrequently.

4. Interstellar and Interplanetary Matter

Components of the interstellar medium which enter the heliosphere from deep spaceinclude neutral gas atoms, larger interstellar dust grains, and galactic cosmic rays. Theproducts created by the interactions of the ISM and solar wind create an ISM-dominated

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6 Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material

Figure 3. Heliosphere predicted for a Sun immersed in the hot Local Bubble (left) andimmersed in a nH=15 cm−3 diffuse cold cloud (right). Figure from Muller et al. (2002).

heliosphere. Fig. 4 shows an overview of the heliosphere, with the products of the inter-action between the ISM and solar wind identified.

4.1. High Energy Galactic Cosmic Rays in the Heliosphere

Galactic cosmic rays with energies less than ∼100 GeV/nucleon are modulated by the in-creasingly nonuniform structure of magnetic fields embedded in the outward flowing solarwind during solar maximum (Fig. 6). The result of this modulation is a well known anti-correlation between the solar activity cycle and the cosmic ray flux at the Earth’s surface.The anti-correlation is illustrated in Fig. 5, which shows neutron monitor counts, fromsecondary particles produced by cosmic ray interactions at the top of the atmosphere,versus the sunspot number. This anticorrelation reflects variations in the heliosphericmodulation of the galactic cosmic ray flux as a function of the solar wind magnetic ac-tivity. Anomalous cosmic rays (see below), formed by accelerated pickup ions, experiencemodulation in the heliosphere similar to GCRs. Most cosmic ray modulation occurs inthe outer part of the heliosphere, so that evidence of CR interactions on meteorites orplanetary surfaces should contain fossil evidence on the heliosphere radius. The helio-sphere varies with the solar cycle, as does cosmic ray modulation. Disorder in the solarwind magnetic field at sunspot maximum corresponds to an increase in cosmic ray mod-ulation, although the heliosphere is smaller than at solar minimum. GCRs are capable ofchanging the flow pattern of the solar wind and the surrounding local ISM provided theparticles’ coupling to the plasma is sufficiently strong. The interstellar cosmic-ray spectraand the diffusion coefficients and cosmic-ray pressure gradients within the heliosphere arenow becoming better understood (e.g. Ip & Axford 1985).

4.2. Raw ISM in the Heliosphere: Ho, Heo

Neutral interstellar H and He atoms enter and penetrate the solar system, and are ionizedby charge exchange with the solar wind or photoionization. A weak interplanetary glowfrom the fluorescence of solar Lyα radiation off of interstellar Ho, and solar 584 A radia-tion off of interstellar Heo, led to the discovery of interstellar matter in the solar system

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Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material 7

Figure 4. Overview of the heliosphere, with termination shock, heliopause, bow shock, andouter and inner heliosheath (HS). Some sample plasma (H+), pickup ion (PU Ion), and solarwind plasma (vHS) trajectories are shown, as well as trajectories of neutral hydrogen (H) comingfrom the interstellar medium (HISM) and experiencing charge exchange (*), and galactic cosmicrays (GCR). The solar and interstellar magnetic fields (B) are sketched (based on a plot by J.R. Jokipii).

in 1971 (Thomas & Krassa 1971, Bertaux & Blamont 1971, Weller & Meier 1974). Ho isionized at ∼ 4 AU by charge exchange with the solar wind and photoionization, while Heo

penetrates to ∼0.4 AU before becoming photoionized. The flux of Heo atoms has beenmeasured directly by Ulysses, yielding values n(Heo)=0.014±0.002 cm−3, temperature6,500 K, and velocity of 26.4 km s−1 and and upstream direction lII=3.3o, bII=+15.9o

(Witte et al. 1996 and private communication). The first spectral observations of in-terstellar Ho in the solar system observed a projected velocity –24.1±2.6 km s−1 duringsolar minimum towards the direction lII=16.8o, bII=+12.3o (Adams & Frisch 1977). Cor-recting this velocity towards the Heo upstream direction gives a cloud velocity 24.8±2.6km s−1, in agreement with the Heo velocity (since during solar minimum radiation pres-sure and gravity are approximately equal). The LSR upstream direction of the LIC islII∼346o, bII∼–1o.Interstellar Ho and Heo behave differently in the heliosphere. About 20%–40% of the

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8 Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material

Figure 5. Solar cycle modulation of >3 GeV galactic cosmic rays: Sunspot num-ber versus modulated galactic cosmic ray intensity. This figure is also available athttp://ulysses.uchicago.edu/NeutronMonitor/neutron mon.html along with related data.

Ho is lost in the outer heliosheath through charge-exchange with interstellar H+, andonce in the solar system theHo trajectory is governed by the relative strengths of the solarLyα radiation pressure force and gravity. Interstellar Heo passes through the heliosheathunaltered, and the trajectory in the heliosphere is governed by gravity so that interstellarHe is gravitationally focused downstream of the Sun. The Earth passes through the Hefocusing cone about December 1 of each year. The Heo cone density is enhanced at 1 AUby a factor of ∼250 over the value at infinity, but the peak density of the focusing coneis inside 1 AU (Michels et al. 2002).

4.3. Raw ISM in the Heliosphere: Dust

Interstellar dust grains (ISDG) with radii >0.2 µm enter the heliosphere and have beendetected by instruments on board Ulysses, Galileo, and Cassini (e.g. Baguhl et al. 1996,Frisch et al. 1999, Landgraf 2000). The mass flux distribution of these grains is shownin Fig. 7. Smaller grains (< 0.1 µm) are deflected in the heliosheath region and do notenter the heliosphere.Large ISDGs (radii >0.35 µm) are focused downstream of the Sun, in a prominent

gravitational focusing cone which is more extensive than the He focusing cone, extendingover 10 AU in the downstream direction (Landgraf 2000). Large ISDGs constitute ∼30%of the interplanetary grain flux with masses > 1013 gr (or radius> 0.2 µm) at 1 AU(Gruen & Landgraf 2000).ISDGs in the size range comparable to classical dust particles (0.1-0.2 µm, charge ∼1

eV) show a distribution in the heliosphere which varies with time because of Lorentzcoupling to a solar wind magnetic field which changes in polarity every 11-year solarcycle. These positively charged grains alternately are focused and defocused towards the

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101

102

103

104

T, MeV

10-2

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100

101

102

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ster

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Figure 6. An example of the modulated cosmic ray spectrum at different locations in theheliosphere. The dashed line shows the modulated proton spectrum in the heliosheath (θ = 00)at 110 AU, the dash-dotted line is for the supersonic solar wind at 10 AU, and the dotted lineis for the heliotail (θ = 1800) at 650 AU. The unmodulated interstellar spectrum is shown as asolid line. Experimental data from BESS (squares) and IMP8 (circles) are shown for comparison.Figure from Florinski et al. (2002). (At 102 MeV, from top to bottom the lines are: solid, dotted,dashed, dot-dashed.)

ecliptic plane. The 1996 solar minimum corresponded to a defocusing phase (Landgraf2000).The gas-to-dust mass ratio (Rgd) in the LIC is Rgd=125+18

−14, based on comparisonsbetween interstellar dust in the solar system and the properties for the gas in the LIC,or Rgd=158 based on missing mass arguments (Frisch & Slavin, 2002).Radar measurements of micrometeorites show sources from outside the solar system.

Interstellar micrometeorites with masses ∼10−7 g are detected by radar observationsof the atmospheric trajectories and velocities (Baggaley 2000, Landgraf et al. 2000).A discrete source is seen at the location of β Pic (determined after solar motion isremoved). These observations from the southern hemisphere also show an enhanced fluxfrom the southern ecliptic. In the northern hemisphere, Doppler radar measurementsof micrometeorites provide evidence for a radiant direction towards the Local Bubble(Meisel et al. 2002).

4.4. Solar Wind-ISM Interactions Products: Pickup Ions and Anomalous Cosmic Rays

Interstellar atoms with first ionization potentials &13.6 eV enter and penetrate the solarsystem, and are ionized by charge exchange with the solar wind. The resulting ions arecoupled to the solar wind by the Lorentz force, where they are observed as a populationof pickup ions (PUI, Gloeckler and Geiss 2002). PUIs of H, He, N, O, and Ne provide adirect sample of ionization levels in the LIC (Slavin & Frisch 2002). PUIs are acceleratedto cosmic ray energies in the region of the termination shock of the solar wind, forming ananomalous population of cosmic rays (Garcia-Munoz et al. 1973, McDonald et al. 1974,Fisk et al. 1974). Anomalous cosmic rays, which are “anomalous” because of composition

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10 Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material

10-18 10-16 10-14 10-12 10-10

m [kg]

10-10

10-8

10-6

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flux

[m-2s-1

]

Ulysses/Galileo data

AMOR southern ecl

AMOR discrete source

AMOR upwind direction

Figure 7. Mass flux of interstellar dust grains observed within the solar system by the Ulysses,Galileo and Cassini spacecraft (Baguhl et al. 1996, Landgraf et al. 2000). The AMOR radardata points are of extrasolar micro-meteorites, and the point source corresponds to a directiontowards β Pic (Baggaley 2000).

and energy, typically have lower energies than galactic cosmic rays. The anomalous cosmicray H, He, N, O, Ne, and Ar populations have an interstellar origin, and thus providean additional tracer of the neutral species in the LIC (Cummings and Stone 2002).Anomalous cosmic rays with energies >1 MeV/nucleon and an interstellar origin are alsofound trapped in the radiation belts of the Earths magnetosphere (e.g. Adams & Tylka1993, Mazur et al. 2000).

5. Astrospheres and Extrasolar Planetary System

An astrosphere is the stellar wind bubble around a cool star. Cool stars with stellarwinds will have astrospheres regulated by the physical properties of the interstellar cloudsurrounding each star (Frisch 1993), and stellar mass loss properties can be inferred fromHo Lyα absorption formed in the hydrogen wall region in the compressed heliosheathgas (Wood et al. 2002). The nearest star α Cen AB (1.3 pc) has a mass loss rate ∼ 2times greater than the solar value (Wood et al. 2001). The pileup of interstellar Ho

in the nose region of astrospheres surrounding nearby cool stars (e.g. α Cen, ǫ Eri, 61CygA, 36 OphAB, 40 Eri A, Gayley et al. 1997, Wood et al. 2002), indicates that othercool stars have astrospheres which can be modeled using methodology developed for theheliosphere.The astrosphere configuration for extrasolar planetary systems will vary with the in-

dividual properties of each system. The Sun moves through the local standard of restwith a velocity of V∼13 km s−1, but many cool stars have larger velocities. Typical dif-fuse interstellar clouds move through space with velocities 0–20 km s−1 (or more), andthe dynamical ram pressure (∼V2) may vary by factors of ∼103, and cause variationsin the astrosphere radius of factors of >30. The result is that inner and outer planetsof extrasolar planetary systems will be exposed to different amounts of raw interstellarmatter over the lifetime of the planetary system. Frisch (1993) estimated astrosphere

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Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material 11

Figure 8. Locations of ∼40 extrasolar planetary systems in galactic longitude and latitude.The plotted numbers are the star distance. The regions marked “High N(H)” show the upstreamdirection of the cluster of local interstellar clouds, towards which stars within ∼30 pc are likelyto be embedded in a diffuse interstellar cloud. The direction towards “Low N(H)” shows thedirection towards the interior of the Local Bubble or the north pole of Gould’s Belt, towardswhich stars beyond ∼5 pc are more likely to be embedded in the hot gas of the Local Bubbleor high-latitude very low density ISM. The N(H) regions are based on Genova et al. (1990).

radii and historical galactic environments of ∼70 G-stars within 35 pc of the Sun fromthe basic Axford-Holzer equation using the correct stellar dynamics, a solar-like stellarwind, and a realistic guess for the cloud properties. However, this primitive approachcan now be improved upon with sophisticated multifluid astrosphere models (e.g. Zank1999), improved data from the Hipparcos catalog, and improved understanding of thenearby ISM.Astrosphere models, based on self-consistent algorithms for the coupling of interstellar

and secondary neutrals and ions through charge exchange, predict observable signaturesof the interaction of stellar winds and the ISM. The interaction products contain severaldistinct populations which trace both ISM kinematics and the underlying donor plasmapopulation. Comparisons between predictions of global astrospheric models and Lyαabsorption lines towards nearby cool stars demonstrate that external cool stars haveastrospheres with detectable hydrogen walls.The modulation of GCRs and ACRs in the heliosphere indicates that the cosmic ray

fluxes in an astrosphere will depend on the characteristics of the stellar wind interactionwith the surrounding interstellar cloud. Stellar activity cycles give information on themass loss from external cool stars. Activity cycles are observed towards many G-stars,although true solar analogues are not obvious (e.g. Baliunas & Soon 1995, Henry et al.2000).The galactic positions and distances of ∼40 nearby planetary systems are shown in Fig.

8. The same figure illustrates the asymmetric distribution of interstellar matter within

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12 Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material

∼35 pc of the Sun, with most of the material located in the upstream direction towardsthe galactic center (labeled “High N(H)”) and very little ISM in the downstream direction(towards the interior of the Local Bubble, “Low N(H)”) or near the North Pole (“LowN(H)”). Stars beyond ∼5 pc towards low-N(H) directions are likely to be embedded inthe Local Bubble, while stars within ∼40 pc in the high-N(H) directions are likely to bein diffuse clouds (which may have densities of up to several particles cm−3). By analogywith the Sun, the galactic environments of extrasolar planetary systems will change withtime.

6. Connections between Astrospheres and Planetary Climates

Building on the knowledge that the Sun is receding from the constellation of Orion,an area of active star formation and giant molecular clouds, Shapley (1921) speculatedthat the ice ages on Earth resulted from a solar encounter with the molecular cloudsin Orion. Since this earliest speculation, there have been a number of attempts to linkcosmic phenomena and the terrestrial climate. The investigated phenomena include (butare not limited to) studies of encounters with molecular clouds that may be in spiral arms(Thaddeus 1986, Scoville & Sanders 1986, Innanen et al. 1978, Begelman & Rees 1976,McCrea 1975, Talbot & Newman 1977), changes in atmosphere chemistry due either toenergetic particles from supernova or the accretion of ISM (Brakenridge 1981, McKay& Thomas 1978, Butler et al. 1978, Fahr 1968), nearby supernova (Sonett et al. 1987,Sonett 1997), or variations in the global electrical circuit or tropospheric cloud cover fromcosmic ray flux variations in the atmosphere (Roble 1991, Rycroft et al. 2000, Tinsley2000, Marsh & Svensmark 2000).Marsh and Svensmark (2000) presented plausible evidence that a correlation is present

between cosmic ray fluxes and low altitude (<3.2 km) cloud cover, which they attributeto cloud condensation around ionized aerosol particles. They also argue that low opticallythick clouds cool the climate. The correlation was observed for low altitude clouds overthe 1980–1995 interval, and the correlation is dominated by a cosmic ray flux minimumcorresponding to the ∼1991 solar maximum, using Huancayo neutron counts (cutoffrigidity 13 GeV) as the cosmic ray monitor. This correlation, apparently related to waternucleation on ionized aerosols, provides a possible mechanism for an astrosphere-climateconnection which can be quantitatively evaluated.The evolution of advanced life has occurred while the Sun was immersed in the vacuum

of the Local Bubble, and the anomalous cosmic ray population inside the heliospherewould have nearly vanished and the enlarged heliosphere would have yielded an effectivecosmic ray modulation (Mueller et al. 2002). Such a galactic environment may havepromoted stability in the terrestrial climate.

7. Conclusions

The evolution of advanced life has occurred during a time when the Sun was immersedin the vacuum of the Local Bubble, so that the enlarged heliosphere would have yieldedeffective modulation of galactic cosmic rays. In contrast, an encounter with a modestdensity diffuse cloud (n(HI) ∼10 cm−3) is possible within 104 – 105 years, and woulddestabilize the heliosphere and modify cosmic ray fluxes impinging on the Earth. Themodulation of both galactic and anomalous cosmic rays by solar wind magnetic fields, andthe emerging link between cosmic ray fluxes and climate forcing, suggests that a stableheliosphere, and by analogy stable astrospheres, are significant factors in maintainingclimatic stability as is necessary for sustainable civilization.

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Frisch, Muller, Zank, and Lopate: ISM and Interplanetary Material 13

The Galactic environment of a star determines interplanetary medium properties, in-cluding the distribution of cosmic rays in the astrosphere. How does this affect the “As-trophysics of Life”, which is the topic of this conference? Over the past century manysuggestions have been made regarding Galactic effects on Earth’s climate. Recent workhas demonstrated that the global electrical circuit is moderated by the cosmic ray flux(Roble 1991), and that, for instance, cloud cover in the lower troposphere (<3.2 km)correlates with cosmic ray flux (Marsh & Svensmark 2002). The fact which is clear,however, is that at the present time the solar wind shields the Earth from most ISMproducts. Relatively low fluxes of energetic particles, including galactic cosmic rays (>1GeV/nucleon) and anomalous cosmic rays (<0.5 GeV/nucleon), are able to penetrate tothe Earth however.

Simulations which describe the interaction between interstellar clouds and stellar windswill provide valuable information on the properties of the astrospheres of extrasolar plan-etary systems, as well as a basis for evaluating the interplanetary environment. Under-standing the historical properties of astrospheres around extrasolar planetary systemswill provide a basis for evaluating the climatic stability on possible Earth-like extrasolarplanets. The differences in exposure to raw ISM for inner and outer planets over theplanet lifetimes may be significant.

acknowledgmentsPCF would like to thank NASA for research support through grantsNAG5-8163, NAG5-1105, and NAG5-6405 to the University of Chicago. GPZ and HRMacknowledge the partial support of an NSF-DOE grant ATM-0296114 and NASA grantNAG5-11621. CL thanks and acknowledges NSF grant ATM 99-12341 for providing sup-port for the cosmic ray research.

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